Concrete Calculator: How Many Bags Do I Need?
To order concrete, work out the volume of the pour in cubic feet (slab: length × width × thickness in feet; footing: run × width × depth; post hole: π × radius² × depth), add a waste allowance, then divide by the yield printed on the bag. Quikrete and Sakrete both list 0.6 cu ft per 80 lb bag, 0.45 cu ft per 60 lb bag and 0.3 cu ft per 40 lb bag. One cubic yard is 27 cubic feet, which is 45 bags of 80 lb mix.
Concrete calculator
Pick what you are pouring and enter its size in feet and inches. Results update as you type and are saved in the page address so you can bookmark them. Nothing leaves your browser.
How the concrete calculator works
The calculator turns your measurements into cubic feet, adds your allowance, then converts to cubic yards (divide by 27) and to bags (divide by the yield on the bag, rounding up). It shows all three bag sizes so you can compare what your store stocks.
slab = length (ft) × width (ft) × thickness (in) ÷ 12 footing = run (ft) × width (in) ÷ 12 × depth (in) ÷ 12 post holes = count × π × (diameter ÷ 2)² × depth ÷ 1728 (all in inches) cubic yards = cubic feet × (1 + waste ÷ 100) ÷ 27 bags = round up (cubic feet × (1 + waste ÷ 100) ÷ yield per bag)
- yield
- cubic feet of mixed concrete per bag: 0.3 (40 lb), 0.45 (60 lb), 0.6 (80 lb)
- 1728
- cubic inches in a cubic foot
- waste
- allowance in percent for spillage and an uneven base
Worked example
Given: a 10 ft × 10 ft patio slab; 4 inches thick; 10% waste allowance; 0.6 cu ft per 80 lb bag.
- Volume:
10 × 10 × 4 ÷ 12 = 33.33 cu ft - With the allowance:
33.33 × 1.10 = 36.67 cu ft - Cubic yards:
36.67 ÷ 27 = 1.36 cu yd - 80 lb bags:
36.67 ÷ 0.6 = 61.11 → 62 - 60 lb bags:
36.67 ÷ 0.45 = 81.48 → 82 - 40 lb bags:
36.67 ÷ 0.3 = 122.22 → 123
Answer: 1.36 cubic yards: buy 62 bags of 80 lb mix (or 82 × 60 lb, or 123 × 40 lb). Without the allowance the 80 lb count is 56.
Published bag yields
| Bag size | Yield (cu ft) | Bags per cubic yard | Source |
|---|---|---|---|
| 80 lb | 0.6 | 45 | Sources 1 and 2 |
| 60 lb | 0.45 | 60 | Sources 1 and 2 |
| 40 lb | 0.3 | 90 | Sources 1 and 2 |
Bags per cubic yard is 27 ÷ yield (source 5), before any waste allowance. Quikrete lists the 40, 60 and 80 lb sizes for Concrete Mix No. 1101; Sakrete’s data sheet gives the same yields.
Slab quick reference
| Slab area | 4 inch slab | 6 inch slab |
|---|---|---|
| 50 sq ft | 0.62 cu yd · 28 bags | 0.93 cu yd · 42 bags |
| 100 sq ft | 1.23 cu yd · 56 bags | 1.85 cu yd · 84 bags |
| 200 sq ft | 2.47 cu yd · 112 bags | 3.7 cu yd · 167 bags |
| 400 sq ft | 4.94 cu yd · 223 bags | 7.41 cu yd · 334 bags |
Computed with the formula above and the 80 lb yield. Quikrete’s own calculator works the same way for 4 and 6 inch slabs (source 3).
Assumptions and limits
- Standard bagged concrete mix. Fast-setting, high-yield and specialty mixes yield differently; use the figure on your bag.
- Slabs and footings are rectangles of even thickness; post holes are straight cylinders filled to the top. A footing with steps or a sloping slab needs to be split into parts.
- The 10% allowance is a planning default, not a manufacturer figure. Quikrete says its yields allow nothing for waste or an uneven base (source 3).
- Footing and slab sizes are your inputs. Frost depth, footing width and slab thickness are set by your plans and local building code, not by this calculator.
- Reinforcement, gravel base, forms and curing materials are not included.
- Manufacturer figures are as published on 2026-11-13 and can change.
Frequently asked questions
How many 80 lb bags of concrete do I need for a 10 x 10 slab?
A 10 x 10 ft slab 4 inches thick is 33.33 cubic feet. At 0.6 cu ft per 80 lb bag that is 55.56 bags, so 56 bags with no allowance. With a 10% waste allowance it is 36.67 cu ft, or 62 bags. A 6 inch slab of the same size needs about half as much again.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. Using the published yields that is 45 bags of 80 lb mix (27 ÷ 0.6), 60 bags of 60 lb mix (27 ÷ 0.45) or 90 bags of 40 lb mix (27 ÷ 0.3). Because every bag count rounds up, a real order is a little higher.
How much concrete do I need for a fence or deck post?
Quikrete’s Fast-Setting Concrete data sheet says the hole should be 3 times the post diameter and as deep as one-half of the post height above ground. A 4x4 post (3.5 inch actual) with a 10.5 inch hole and a 36 inch deep hole takes about 1.8 cubic feet of concrete, a little over three 80 lb bags, per post. Enter your own post count, hole diameter and depth in the calculator. For holes deeper than 30 inches the data sheet says to place the material in lifts of 30 inches or less.
How thick should a concrete slab be?
It depends on the use and your local code. Quikrete’s own bag calculator offers 4 inch and 6 inch slabs, and says its Concrete Mix is for pours 2 inches thick or more. For a driveway, a shed floor or anything carrying vehicles, ask your building department or follow your plans.
Should I add extra for waste?
Yes. Quikrete says its bag yields are approximate and do not allow for an uneven base or waste. The calculator starts at 10%, which is our planning default, not a manufacturer figure. Spillage, an uneven subgrade and over-dug post holes all use extra. Raise it for rough ground or a first pour.
Do all concrete mixes have the same yield?
No. The yields here are for standard concrete mix: Quikrete Concrete Mix and Sakrete High-Strength Concrete Mix print the same figures. Fast-setting mix in a 60 lb bag also lists 0.45 cubic feet. High-yield, rapid-strength and specialty mixes differ, so use the yield printed on the bag you buy and divide the cubic feet from the calculator by it.
Is it cheaper to mix bags or order ready-mix?
We do not publish prices, so use your local quotes. The calculator gives you the volume in cubic yards, which is how ready-mix is ordered, and the bag counts, so you can price both. Ask the ready-mix supplier about its minimum load and delivery fee, and remember that mixing dozens of bags by hand is a long, heavy job.
Does the calculator include rebar, gravel base or forms?
No. It only works out the concrete. Reinforcement, a compacted gravel base, forms, a vapor barrier and curing supplies are separate, and your plans or local code decide what they need to be. For the gravel base, use the gravel calculator and enter the area and depth.
Sources
- Quikrete, Concrete Mix No. 1101 product data (Construction Products Guide): bag sizes and yield (quikrete.com) Accessed 2026-11-13.
- Sakrete, High-Strength Concrete Mix technical data sheet: yield by package size (sakrete.com) Accessed 2026-11-13.
- Quikrete, Concrete Calculator (4 inch or 6 inch slab; yields approximate, no allowance for waste) (quikrete.com) Accessed 2026-11-13.
- Quikrete, Fast Setting Concrete Mix No. 1004-60 data sheet: setting posts and yield (quikrete.com) Accessed 2026-11-13.
- Texas A&M AgriLife Extension, Travis County: How Much Compost, Soil or Mulch? (27 cubic feet per cubic yard) (travis-tx.tamu.edu) Accessed 2026-11-13.